Optical receiver with extended source discrimination

ABSTRACT

An optical receiver comprising a lens system for forming an image of a distant light source at its focal plane. The light on either side of the focal plane is intercepted by a reticle of alternate opaque and transparent strips of equal widths which move at constant speed and always in the same direction across the light while the reticle continuously oscillates back and forth along the optical axis between extreme positions one of which is in the focal plane of the lens. A photodetector converts the light transmitted by the reticle into a proportionate electrical signal. An inhibit gate located between the output of the photodetector and the receiver output circuit is made to respond to the presence of an amplitude modulation of the AC component of the photodetector output at either the oscillating frequency of the reticle or twice this frequency to prevent an output from the receiver when either of these modulations is present.

United States Patent 1191 Thomas OPTICAL RECEIVER WITH EXTENDED PrimaryExaminer-Walter Stolwein SOURCE DISCRIMINATION Attorney-Harry A.Herbert, Jr. et al.

[75] Inventor: Charles T. Thomas, Dallas, Tex. [57] ABSTRACT AssisneerThe United Slates Amflicfl An optical receiver comprising a lens systemfor form- "P y "h Secretary of the ing an image of a distant lightsource at its focal plane. Washmglon. The light on either side of thefocal plane is inter- [22] Filed; No 22 1972 cepted by a reticle ofalternate opaque and transparent strips of equal widths which move atconstant [21] PP N05 3081888 speed and always in the same directionacross the light while the reticle continuously oscillates back andforth 52 us. c1. 250/203 R, 250/233, 250/342 ahmg P axis between extremePositionS one 151 111:. C1. G0lj 1/20 Of which is in the focal P thelens- A pholode- [53] n w f Search 250/203 R, 233, 334, tector convertsthe light transmitted by the reticle into 250/342 a proportionateelectrical signal. An inhibit gate located between the output of thephotodetector and 5 References Cited the receiver output circuit is madeto respond to the presence of an amplitude modulation of the AC com- 2 9588 f T J PATENTS 250/203 X ponent of the photodetector output at eitherthe oscil- 1cox, r 3.220.298 11/1905 Powell =1 al. 250/233 x iz jzii fii 32s fregiluenc f 3,272,985 9/1966 Hayes 250/203 x p l r o 3.411;,47712/1968 Falbel 250 342 x these amns present 3 Claims, 6 Drawing Figures(1-) 9 (b g (a) I; 1 Pflara- [9c 1 CNoPP/A/ 77VSfldt0 I F'Nl/INC g 0/8 c0/7 {067417049 9 L aavo'ouzaru A: l r l8 7 5 #0:: I/tfl [EC 77!! 6ef/l/ffl'l't 0 hwy/317" 6 if c/zcwr. ear-6 1 OI'I'ICAL RECEIVER WITHEXTENDED SOURCE DISCRIMINATION BACKGROUND OF THE INVENTION Thisinvention pertains to the optical detection of distant luminous sourceswhich appear small when viewed from the detector and are generallyreferred to as point sources. In particular, the invention is concernedwith point source detectors that are capable of discriminating againstextended light sources, or sources that appear relatively large whenviewed from the detector. An example would be the detection of a distantapparently small light source against a background of sunlit clouds.

Optical receivers for the detection of point sources are known whichseek to discriminate against extended sources by means of a choppingreticle. The chopping reticle is usually a rotating disc having at itsedge adjacent narrow radial strips that are of equal widths andalternately transparent and opaque. The disc is positioned in the focalplane of the receiver lens so that an image of the distant source isformed on the reticle. A photoelectric detector converts the lightpassing through the reticle into an electric current that has analternating component due to the chopping of the light by the alternatetransparent and opaque strips. When the size of the image is equal to orless than the width of the strips all of the light is chopped and theamplitude of the alternating component equals the average value of thedetector output current. As the image size exceeds the width of thestrips, the fraction of the total light in the image that is choppeddecreases with the result that the ratio of the amplitude of thealternating component to the average value of the signal decreases,reaching a value of zero at an image size that is twice the width of arecticle strip. As the image size increases beyond this point, the ratiorises toward a new maximum at an image size three times the reticlestrip width, which maximum however has a value of only one-third.Similarly the ratio continues to pass through values of zero at thehigher even multiples of the reticle strip size and through decreasingmaxima at the higher odd multiples of the strip size, as will bedescribed in more detail later. A system of this type provides a certaindegree image size discrimination in the above described dependency ofthe ratio of the alternating component of the photodetector outputcurrent to the average value of the current on image size. Since thisratio has its maximum value of unity for images equal to or smaller thana reticle strip width and rapidly decreases for images larger than areticle strip width, a threshold device may be used to produce an outputfrom the receiver only when the image size does not exceed the stripdimension. However, this requires the complication of apparatus forderiving the ratio of the alternating component of the photodetectoroutput current to its average value. More simply, the alternatingcomponent alone may be measured but this gives ambiguous results sincethe amplitude of this component depends both upon the image size and thetotal light flux in the image.

SUMMARY OF THE INVENTION The purpose of the invention is to provide anoptical receiver for the detection of distant light sources whichproduces no output if the image of the light source exceeds apredetermined size regardless of the total light flux in the image.

Briefly, this result is accomplished by the use of an optical systemwith a rotating reticle as described above in which a slight wobble isimparted to the reticle so that the reticle surface coincides with thefocal plane of the lens at only one point in its rotational cycle. Wherethe size of the image of the light source on the reticle does not exceedthe width of a reticle strip even in its most defocused state, i.e.,when the reticle has its maximum displacement from the focal plane, allof the light is chopped throughout the rotational cycle of the reticleand the ratio of the alternating component to the average value of thedetector output is unity throughout the cycle. Under this conditionthere is no amplitude modulation of the detector output at therotational frequency. On the other hand, if the image size is such thatit exceeds the reticle strip width during part or all of the rotationalcycle the ratio of the alternating component to the average value of thedetector output varies during the cycle. This causes an amplitudemodulation of the detector output at the rotational frequency or, insome special cases, at twice the rotational frequency. An inhibitingcircuit responsive to these two frequencies is used to prevent an outputfrom the re ceiver whenever either of these frequencies is present inthe detector output. In this manner the receiver pro duces an outputonly for light sources having images less than a size determined by thereticle strip width. By making the reticle dimension small enough, thereceiver will discriminate against all but essentially point sources."

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an example of the prior art,

FIG. 2 shows the construction of a chopping reticle,

FIG. 3 shows the effect of image size relative to reticle strip width ofthe photodetector output current,

FIG. 4 is a graph showing both the fraction of light chopped and therelative response versus the ratio of the image size L to the reticlestrip width d,

FIG. 5 shows an optical receiver incorporating the invention, and

FIG. 6 shows waveforms occun'ng in the receiver of FIG. 5.

DETAILED DESCRIPTION Referring to the prior art system shown in FIG. I,light from a distant source is focused by lens I on the surface of disc2 which surface has on it a reticle of alternate transparent and opaquesectorial strips 3 and 4 of equal widths, as seen in FIG. 2. The disc isrotated at constant speed by motor 5. Light passing the reticle iscollected by photoelectric detector 6 which produces an output currentin load resistor 7 proportional to the total light flux.

The effect of the size of the image relative to the width of the reticlestrips may be seen in FIG. 3. In this figure the images are shown assquares for simplicity. Further, the total light flux constituting theimage is the same for all images, as would be the case where thesmallest image 8 represents the properly focused image at the focalplane of lens I and the other images represent various degrees ofdefocus of the light source image occuring at increasing distances fromthe focal plane. As will be seen later, this is the situation thatexists in the invention. In the drawing the image is shown in solidoutline when its position relative to the reticle provides maximum lighttransmission and in dotted outline when its position provides minimumlight transmission.

The average value of light transmitted by the reticle and reaching thephotodetector is constant and independent of the ratio L/d, where L isthe width of the square image and d is the width of a reticle strip. Themaximum and minimum values of light transmitted, however, depend uponthe value of L/d. When this ratio is equal to or less than unity themaximum value of light transmitted equals total light in the image andthe minimum value is zero, or, in other words, all of the light ischopped by the reticle. This gives rise to a photodetector outputcurrent that alternates, like the transmitted light, between maximum andzero values, in which case the amplitude of the alternating component ofthe current equals the average value of the current represented in FIG.3 by the dotted line. The ratio of the alternating component of thephotodetector output current to the average value of the current,designated the relative response of the receiver, in this case ismaximum and is designated l which also represents the fraction of thetotal light in the image that is chopped by the reticle. This situationis represented in FIG. 3 for values of L/d equal to 0.6 and 1.0. As thevalue of L/d increases above unity the maximum and minimum values oflight transmitted respectively decrease and increase equally toward theaverage value thus reducing the amplitude of the alternating componentwithout changing the average value of the detector output. An example isgiven in FIG. 3 for L/d 1.2, for which the relative response andfraction of light chopped have fallen to 0.67. This process continuesuntil at L/d 2 the maximum and minimum values of transmitted light anddetector output are both equal to the average value and the alternatingcomponent is zero, as illustrated in the drawing. Above L/d 2, themaximum and minimum values of transmitted light and detector outputincrease and decrease respectively from the average value giving anincreasing value of relative response until a maximum is reached at L/d3. An example at L/d 2.6, for which the relative response is 0.23, isshown in the drawing. FIG. 4 is a continuous graph showing relativeresponse as a function of L/d for values of the latter from below unityabove 6. As seen in the graph, the relative response is zero at valuesof L/d equal to the even integers and has decreasing maxima at values ofL/d equal to the odd integers.

The prior art receiver of FIG. 1 operates in the simpler of the twomodes previously mentioned, i.e. it does not derive the ratio of the ACcomponent of the photodetector output to the average value of the outputbut measures the AC component only. The AC- component is applied throughcapacitor 9 to AC amplifier 10 the response of which is peaked at thechopping frequency. The chopping frequency in hertz is equal to theproduct of the number of transparent or opaque strips in the reticle andthe speed of disc 2 in revolutions per second. Rectifier 11 produces adirect voltage proportional to the amplitude of the amplifier 10 output.Threshold circuit 12 produces an output only when the rectifier outputmagnitude exceeds a preset threshold. As seen in FIGS. 3 and 4 thealternating component drops off rapidly as the image size exceeds thereticle strip width. Therefore, the threshold can be adjusted so thatthe receiver produces an output only when the image size does notappreciably exceed the strip width. As stated earlier, since theamplitude of the alternating component is affected by the total lightflux in the image as well as by image size relative to reticle stripsize, the receiver of FIG. 1 gives unambiguous results only when thelight flux is equal in all images, the situation illustrated in HO. 3.

The optical receiver of FIG. 5, constructed in accorance with theinvention, produces an output only when the image size does not exceed apredetermined size regardless of the total amount of light flux in theimage. In this receiver, the lens system 1 and the disc 2 and associatedreticle are the same as in FIGS. 1 and 2 except that disc 2 is mountedat an angle to the shaft of motor 5 so as to impart a slight wobble tothe disc. The arrangement is such that when disc 2 has its nearest orits farthest position relative to the lens the point at which thereticle is pierced by the optical axis of the lens lies in the focalplane of the lens. In the drawing this occurs at the nearest position,shown in solid outline. later in the rotational cycle the disc is at itsother extreme, shown in dotted outline in the drawing, and the point atwhich the optical axis pierces the reticle has its maximum displacementfrom the focal plane. If the image of the light source on the reticle issmall enough that it does not exceed the reticle strip width in its mostdefocused state, i.e., when the disc 2 has the position shown in dottedoutline in FIG. 5, the ratio L/d does not exceed unity and all of thelight is chopped throughout the rotational cycle. Therefore, as seen inFIGS. 3 and 4, the AC component of the photodetector output is constantand has its maximum value, which is equal to the average value of thesignal, throughout the rotational cycle. In this case the waveforms atpoints (a), (b) and (c) in FIG. 5 are as shown in the point sourceportion of FIG. 6. The demodulator 13 is simply a rectifier and low passfilter which removes the chopping frequency components. The output ofthe demodulator in this case is a direct voltage of constant magnitudethroughout the rotational cycle of the disc 2. This signal passesthreshold circuit 14 and inhibit gate 15 to output circuit 16.Consequently, a light source producing an image that does not exceed thestrip width of the reticle throughout the rotational cycle of disc 2produces a DC output from the receiver and with a sufficiently smallreticle strip width, such outputs may be limited to point sources.

If the light source is large enough that its image size exceeds thereticle strip width during all or a substantial part of the rotationalcycle of disc 2, the ratio L/d will exceed unity during all or part ofthe cycle and, as seen in FIGS. 3 and 4, the amplitude of the ACcomponent will vary with the value of L/d during the cycle. Where theslope of the curve in FIG. 4 does not change sign within the range ofvariation of L/d, the fundamental frequency of this amplitude modulationof the AC component equals the rotational frequency f, of disc 2. Wherethe slope changes sign within the range of variation of L/d, thefundamental frequency is twice the rotational frequency or 2f,.Capacitor 17 blocks the DC component in the output of demodulator l3 andfilter 18 blocks all other components except those at the frequencies f,and 2]}. lf either or both of these components are present, there is aDC output from rectifier 19 which is amplified in amplifier 20 andapplied through threshold circuit 21 to inhibit gate to close this gateand thereby prevent the output of threshold circuit 14 from reachingreceiver output circuit 16. The waveforms at points (a)-(e) for asituation where the ratio U1! is greater than unity throughout therotational cycle and the slope of the curve in FIG. 4 does not changesign over the range of variation of L/d are illustrated in the extendedsource" portion of FIG. 6.

The output of the optical receiver in PK]. 5 is therefore inhibitedwhenever the light source image is large enough to produce an amplitudemodulation at f, or 2f, of the AC component of the photodetector outputin the above described manner. For smaller light sources, ire. lightsources for which L/d does not exceed unity during the rotational cycleof the disc, there is no amplitude modulation produced and the receiveroutput is not inhibited. The receiver as a result discriminates againstlight sources exceeding a predetermined maximum apparent size dependent,for a given lens system, upon the width of a reticle strip. Sourceswhich do not exceed the predetermined maximum size may be furtherdiscriminated as to total received light flux by an appropriate settingof the threshold of circuit 14. Threshold circuit 21 may be used as anoise discriminator to prevent low level spurious signals frominhibiting gate 15.

1 claim:

I. An optical receiver comprising a lens system for forming an image ofa distant light source at its focal plane; a chopping reticle ofalternate opaque and transparent strips of equal widths; means forimparting two simultaneous motions to said reticle, one motionconstituting a continuous oscillation of the reticle back and forthalong the optical axis of the lens between two extreme positions one ofwhich is in the focal plane of the lens, and the other motionconstituting a continuous constant speed movement of said strips in adirection normal to the strips and to the optical axis of the lens; aphotodetector receiving the light transmitted by the reticle andconverting said light into a proportionate electrical signal in itsoutput circuit, said signal having an alternating component due to thelight chopping effect of the reticle; a receiver output circuit; asignal transmission circuit including a normally transmitting gatecircuit coupled between the photodetector output circuit and thereceiver output circuit; and means responsive to amplitude modulation ofthe alternating component of the photodetector output signal at eitherthe oscillation frequency of said article or twice this frequency toinhibit signal transmission through said gate during the presence ofsaid modulation.

2. Apparatus as claimed in claim 1 in which said reticle and said meansfor imparting two simultaneous motions thereto comprise: a disc havingsaid alternate opaque and transparent strips in the form of a band ofscctorial strips at its periphery, said disc being mounted at its centeron a shaft with the angle between the disc and the shaft slightly lessthan in order to impart a wobble to the disc when the shaft is rotated,said shaft being parallel to the optical axis of the lens and spacedtherefrom as required for the optical axis to pass through said band ofstrips, and means for rotating said shaft at constant speed.

3. Apparatus as claimed in claim 2 in which the last named means of saidclaim comprises: a demodulator situated in said signal transmissioncircuit between the photodetector output circuit and said gate circuitfor deriving the amplitude modulation of the alternating component ofthe photodetector output signal; a filter passing only the saidoscillation frequency of said reticle and twice that frequency; meansfor rectifying the output of said filter to produce a direct inhibitvoltage; and means for applying said inhibit voltage to said gate forpreventing signal transmission therethrough in the presence of theinhibit voltage.

1. An optical receiver comprising a lens system for forming an image ofa distant light source at its focal plane; a chopping reticle ofalternate opaque and transparent strips of equal widths; means forimparting two simultaneous motions to said reticle, one motionconstituting a continuous oscillation of the reticle back and forthalong the optical axis of the lens between two extreme positions one ofwhich is in the focal plane of the lens, and the other motionconstituting a continuous constant speed movement of said strips in adirection normal to the strips and to the optical axis of the lens; aphotodetector receiving the light transmitted by the reticle andconverting said light into a proportionate electrical signal in itsoutput circuit, said signal having an alternating component due to thelight chopping effect of the reticle; a receiver output circuit; asignal transmission circuit including a normally transmitting gatecircuit coupled between the photodetector output circuit and thereceiver output circuit; and means responsive to amplitude modulation ofthe alternating component of the photodetector output signal at eitherthe oscillation frequency of said article or twice this frequency toinhibit signal transmission through said gate during the presence ofsaid modulation.
 2. Apparatus as claimed in claim 1 in which saidreticle and said means for imparting two simultaneous motions theretocomprise: a disc having said alternate opaque and transparent strips inthe form of a band of sectorial strips at its periphery, said disc beingmounted at its center on a shaft with the angle between the disc and theshaft slightly less than 90* in order to impart a wobble to the discwhen the shaft is rotated, said shaft being parallel to the optical axisof the lens and spaced therefrom as required for the optical axis topass through said band of strips, and means for rotating said shaft atconstant speed.
 3. Apparatus as claimed in claim 2 in which the lastnamed means of said claim comprises: a demodulator situated in saidsignal transmission circuit between the photodetector output circuit andsaid gate circuit for deriving the amplitude modulation of thealternating component of the photodetector output signal; a filterpassing only the said oscillation frequency of said reticle and twicethat frequency; means for rectifying the output of said filter toproduce a direct inhibit voltage; and means for applying said inhibitvoltage to said gate for preventing signal transmission therethrough inthe presence of the inhibit voltage.